In case of drying a plastic molding material such as pellets or pulverized materials before molding, a method using a ventilation type dryer has been conventionally adopted. The ventilation type drying is such a method that a charge port for a plastic molding material is provided on the dryer and a discharge port is provided under the dryer, heated air heated to predetermined temperature is introduced under the dryer to ventilate upwardly, and a plastic molding material is heated and dried. However, such a ventilation type drying method which discharges heated air introduced in the dryer to factories is not good for environment, therefore, recently there has been a method of drying a plastic molding material by heat transmission and of evacuating generated moisture out of the system by further vacuumizing and such a method is used as a method which does not release heated air introduced in the dryer therefrom. Patent Literature 1 (JP2000-127153-A) discloses a method for promoting drying in which a little amount of purge air is injected in order to discharge moisture generated in a drying hopper from a dryer, and the amount of purge air is designed so as to be able to promote drying while keeping vacuum state in case of vacuum drying.
However, according to such a method of heating a plastic molding material by heat transfer, it is important to heat a plastic molding material by heat transfer as much as possible and to uniformly heat the material. For this purpose, a heat transfer fin is provided and contacting area of a heat transfer fin and a plastic molding material is required to be wide in order to increase area for receiving heat transfer.
When a plastic molding material stands still for a long time while being surrounded with the heat transfer fins in dried state, the surface of the plastic molding material is activated or gas is volatized from the surface of the plastic molding material. As a result, there have been such problems that the plastic molding material comes to adhere to the heat transfer fin and the plastic molding materials come to adhere each other, thereby being unable to be discharged from the lower end, namely the plastic molding material cannot be discharged from the drying hopper, and the plastic molding material cannot be stably discharged.
As a suggestion for improvement, disclosed in Patent Literature 2 (JP-H07-101150-B), a vibrating device, a striking device, or the like is provided outside a dryer and a plastic molding material in the dryer is shaken by vibration and so on from the outside of the dryer and so on. However, vibration and impact on a machine apply burdens the machine and crack it, and makes unable to execute first-in first-out discharge of a plastic molding material, thereby such a method is inappropriate for continuous processing. In addition, in case of vacuum drying, application of vibration and impact causes leak and sufficient effects cannot be obtained.
In another method as disclosed in Patent Literature 3 (PCT publication WO2004/061383), there has been a method in which material is forcibly pulled and the material in the drying hopper is moved when material is not discharged from a dryer after predetermined time, thereby preventing discharging failure caused by adherence. However, in such a method, drying time for a plastic molding material in the drying hopper cannot be controlled in a circulation line for returning the discharged plastic molding material to the dryer, and failure is caused by over-dried materials and energy is wasted.